The retreat from the High Street has become something of a rout, with HMV, Jessops and Blockbuster just the latest of a string of casualties of the shift to online. There is a lot more pain to come, and no one should play down that. But for successful cities such as London, the move to online could turn out to be a blessing in disguise, enabling us to use scarce land better while protecting the bits of retailing at which London excels.